Process of impregnating and drying wood and composition therefor



Patented Aug. 17, 1954 PROCESS OF IMPREGNATING AND DRYING WOOD ANDCOMPOSITION THEREFOR Eduard Farber, Washington, D. 6., assignor to TheFine Hardwoods Association, Chicago, 111.,

a trade association No Drawing. Application December 18, 1952, SerialNo. 326,794

16 Claims.

This invention relates to wood treatment. More particularly, theinvention relates to a composition and method for treating Wood toreduce the volumetric shrinkage of the wood when the moisture contentthereof is lowered. The invention is particularly applicable to reducethe shrinkage of wood in the form of thin sheets such as are obtained bythe slicing or cutting of wood to produce veneer.

In conventional drying processes the normal shrinkage of wood,particularly in the radial direction, reduces the dimensions of the woodby 6 to 9% or more. In many applications it is particularly desirable toreduce this shrinkage as much as possible. It is particularly importantto reduce shrinkage when the species of wood treated or the method ofmanufacture commands a high price and where tight fitting assemblies arerequired to keep their dimensions under varying conditions of service.

Various attempts have been made by the prior art to limit the amount ofshrinkage which wood undergoes on drying. impregnation or othertreatment by chemicals is one of the methods heretofore suggested.Chemicals suitable for the treatment of wood to reduce the shrinkagethereof when subjected to drying operations must be characterized bychemical stability in the wood. Furthermore, such materials must not behydroscopic or demonstrate a tendency to crystallize on the surface ofthe wood. Additionally, such materials must have a high efficiency inrelatively low concentration. Additionally, such materials must beavailable at low cost. The various salts, carbohydrates, polyalcohols,and the like which have heretofore been suggested for stabilizing woodwith regard to dimensional changes caused by drying have notsatisfactorily met the abovedefined prerequisites.

It is a primary object of this invention to provide a composition whichmay be employed to treat wood whereby the volumetric shrinkage of thewood when the moisture content thereof is reduced is limited.

It is an additional primary object of the invention to provide a methodfor treating wood to limit the volumetric shrinkage of the wood when themoisture content thereof is lowered.

In accordance with this invention wood is treated with a mixture of ureaand a material selected from the group consisting of hydroxyacetic acidand the alkali metal and ammonium salts of hydroxyacetic acid to reducethe volumetric shrinkage of the wood when the moisture content thereofis lowered. The aforementioned mixtures of urea and hydroxyacetic acidand hydroxyacetic salts are effective in low concentration to limit thevolumetric shrinkage of the wood when the moisture content of the Woodis lowered. Furthermore, such mixtures are not hydroscopic to anyappreciable extent and do not impart hydroscopic properties to the wood..Additionally, the mixtures of this invention are stable in the wood andare available at low cost in large quantity.

I-lydroxyacetic acid and the alkali metal and ammonium salts thereofincluding specifically sodium, potassium, lithium, and ammonium salts,are contemplated for utilization in conjunction with urea in thisinvention. Such salts may desirably be obtained by forming first anaqueous solution of urea and hydroxyacetic acid and thereafter adding analkali metal hydroxide or carbonate, such as ammonium, potassium, orlithium hydroxide or carbonate or bicarbonate to the solution so formedin an amount requisite to raise the pH to from about 5 to about 7Mixtures in all relative proportions of urea and hydroxyacetic acid andthe ammonium and alkali metal salts of hydroxyacetic acid arecontemplated by this invention. Especially suitable mixtures containfrom about 0.08 to about 0.30 parts by weight of hydroxyacetic acid orthe alkali metal or ammonium salts of hydroxyacetic acid per part byweightof urea. A preferred range from about 0.16 to about 0.20 parts byweight of hydroxyacetic acid or ammonium or alkali metal salts ofhydroxyacetic acid per part by weight of urea.

The compositions of this invention may be utilized in the form ofsolutions, such as aqueous solutions, as substantially dry crystals, andas a mixture of crystals and water, and applied to the wood by brushing,dipping, immersion, or any other method suitable to eiiect contact ofthe wood with the compositions of the invention.

Appropriate solutions useful for treating wood in accordance with thisinvention comprise aqueous solutions containing from about 10% to about45% by weight, preferably from about 25% to about 35% by weight, of themixture of urea and hydroxyacetic acid and the ammonium or alkali metalsalts of hydroxyacetic acid, as above-defined. Such solutions may beappropriately utilized by dipping the wood to be treated therein for aperiod of from about 3 to about 30 minutes with the solution maintainedat a temperature of from about 15 to about C. Greater or lessertemperatures may be utilized when circumstances indicate that suchmodifications may be expedient.

In many instances, particularly when it is desirable to precludeabsorption by the wood treated of substantial quantities of water fromthe compositions of this invention it is advantageous to apply thcompositions of the invention in 3 more highly concentrated form thanhereinbefore described. In such cases there may be utilized fullysaturated aqueous solutions of the mixtures of urea and hydroxyaceticacid or ammonium or alkali metal salts of hydroxyacetic acidcontemplated by this invention. Concentrated aqueous solutionscontaining from about 45% to about 70% by weight are particularlysuitable. Solutions containing from about 60% to about 70% by weight ofthe urea-hydroxyacetic acid mixtures of this invention are preferablyemployed.

W'hen concentrated solutions as above-defined are utilized the treatmentof the wood may be effected by immersion for a period of from about 3 toabout 30 minutes with the solutions maintained at a temperature of aboutto about 80 C., in like manner to that described with reference to themore dilute solutions. Alternatively, the concentrated solutions may bebrushed on the wood to be treated and the wood thereafter stacked orotherwise stored to permit penetration of the solution in the wood. Liketreatment may be effected with the more dilute solutions previ ouslydescribed.

The amount of moisture present in the compositions of this invention maystill further be reduced by utilizing as the composition to treat'thewood a moist mixture of salts. Such salt mixtures may be prepared bymixing crystalline urea with a technical 70% solution of hydroxyaceticacid or a like solution of the alkali metal or ammonium salts ofhydroxyacetic acid. Preferably crystalline urea is mixed in theproportion of from about 4 to about 7 parts of crystalline urea withfrom about 0.6 to about 1.2 parts of hydroxyacetic acid or an alkalimetal or ammonium salt thereof and about 0.4 to about 0.2 parts of waterby weight. An almost dry mixture of chemicals is thereby obtained whichcan be spread on the thin surface of the sheets of moist wood to permitthe moisture of the wood to dissolve and distribute the same through thewood. In like maner, mere physical mixtures of dry urea and dryhydroxyacetic acid or ammonium or alkali metal salts of hydroxyaceticacid may be utilized.

When such crystalline mixtures of dry salts are utilized it ispreferable that the sheets of wood having the crystals of the saltsspread thereover be stacked together and left for a period of from about1 to 24 hours, at atmospheric temperature, i. e., a temperature of fromabout 5 to about C.

Veneer sheets or like other materials treated are thereafter subjectedto conventional drying operations of the type heretofore practiced inthe art. Such drying operations normally embrace oven drying of the woodat a temperature of from about 200 to 240 F., for a time periodrequisite to reduce the moisture content of the wood to about 5% byweight.

Substantially all species of wood may be treated in accordance with thisinvention, specific reference being made to oak, walnut, gum, poplar,beech, fir, mahogany, and the like. When the invention is particularlyadapted to the wood in the form of thin sheets such as are utilized inthe production of veneer, it may also be applied to wood in any physicalconfiguration which it is desired to treat. Thus railroad ties, andother bulky wood articles may also be treated by this invention. It willbe appreciated by those skilled in the art that the time of treatmentmay of necessity be extended somewhat when other wood 4 articles ofsubstantial thickness are treated in accordance with the method of thisinvention.

Generally speaking, while the invention is applicable to the treatmentof wood having any relative moisture content, the wood treated will becharacterized by a moisture content of 30% to 50% by weight.

It is preferred that the compositions of this invention be utilized inthe method of the invention with a pH not greater than about 7 when thealkali metal and ammonium salts of hydroxyacetic acid are utilized. A pHrange of 5 to 7 is particularly suitable.

EXAMPLE 1 An aqueous solution is prepared containing 6 parts of urea andone part of hydroxyacetic acid and 15 parts of water, all parts being byweight. Walnut veneer having the dimensions of 10.58 wide. thick and amoisture content of about 45% by weight was dipped in this solutionmaintained at a temperature of 60 to 70 0., for a period of about 30minutes. The walnut veneer was thereafter removed from the treatingsolution and the solution was allowed to drain off. The veneer was thendried in a conventional drying oven maintained at a temperature of aboutC., for a period of about 25 minutes. The moisture content thereof wasreduced to about 6% by weight. The width of this walnut veneer sheet wasabout 10.49 inches, thus indicating a shrinkage of less than about 1%. Acontrol walnut veneer sheet identical with that abovedescribed, whichwas not treated in the compositions of this invention but which wasdried in precisely the same manner as that above-described to a moisturecontent of about 6% showed a shrinkage of about 7 to 8%.

A third walnut veneer panel identical with that panel first-describedbut treated in aqueous solution containing 30% by weight of urea for aperiod of 30 minutes while the solution was maintained at a temperatureof 60 to 70 C. and thereafter dried in the same manner as thefirstdescribed sheet to a moisture content of about 6%, was measured andfound to have a width of about 10.30 inches, thereby demonstrating ashrinkage of about 2.7%.

A veneer sheet identical with the one firstdescribed was immersed forabout 30 minutes in a solution containing about 30% by weight of aceticacid, at a temperature of about 60 to 70 C. and thereafter dried to amoisture content of about 6%. The width of this sheet, after drying, wasabout 10.26 inches, thus indicating a shrinkage of about 3%. Thisexperiment was repeated with lactic acid and the dried veneer sheet hada width of about 10.24 inches, thus also indicating a shrinkage of about3%.

The percents of shrinkage above-noted are determined by comparing thewidth of the original veneer sheet of high moisture content with thewidth of the dry sheet and expressing the difference as percentages ofthe original width.

EXAMPLE 2 This example illustrates the practice of the inventionutilizing a more concentrated form of the hydroxyacetic acid-ureamixtures contemplated by the invention.

An aqueous solution was prepared containing 6 parts of urea, 0.70 partsof hydroxyacetic acid, and 4 parts of water, all parts being by weight.This concentrated solution was applied by brushing to the surfaces ofthin sheets of moist walnut wood, containing about 40% by weight, ofwater, having the dimensions of 8.31" Wide, thick. The so-treated sheetswere stacked and left in a stack at a temperature of about to C., for aperiod of about 120 minutes.

The so-treated sheets were thereafter dried in the manner described inExample 1 to a moisture content of about 5%. The width of the sheetsafter drying was about 8.09 inches, thereby indicating a shrinkage ofabout 2.6%.

Control sheets dried in like manner but not treated with thecompositions of this invention demonstrated a shrinkage of about 7.86%.

EXAMPLE 3 together and left for a period of about 24 hours in anatmosphere having a temperature from about 20 to 25 C. The so-treatedsheets were thereafter dried in accordance with the method described inExample 1. The total width of the sheets after drying was about 81.3",thus indicating a shrinkage of only about 1.1%.

EXAMPLE 4-.

This example embraces the results of a series of comparative pilot plantruns.

After slicing, veneer sheets from 3 poplar and 3 gum iiitches wereseparated into two stacks. The veneer sheets were 6 feet in length. Thewidth varied from about 6" to 16". The poplar veneer sheets containedabout by weight of moisture and the gum sheets contained about by weightof moisture.

Some of each of the two stacks of sheets were used as a control and theother was treated with one of the compositions of this invention in themanner indicated in the table.

1 The sheets were immersed in a solution of urea and hydroxyacetic acididentical with that described in Example 1 in the same manner and forthe same period of time and under the same temperature conditions asdescribed in Example 1 and thereafter dried for a period of about fiveminutes in an industrial veneer drying machine, the temperature at theinlet end of which was about 240 E, the temperature at the outlet end ofwhich was about 155 E, to a final moisture content of about 3 to about6% by Weight.

2 The sheets were brushed with a solution of urea and hydroxyacetic acididentical with that described in Example 2 in the same manner and forthe same period of time and under the same temperature conditions asdescribed in Example 2 and thereafter dried for a period of about fiveminutes in an industrial veneer drying machine, the temperature at theinlet end of which was about 240 F., the temperature at the outlet endof which was about 155 F., to a final moisture content of about 3 toabout 6% by weight.

3 The veneer sheets were treated with crystals obtained in the samemanner and under the same conditions as those described in Example 3,and thereafter dried for a period of about five minutes in an industrialveneer drying machine, the temperature at the inlet end of which wasabout 240 F., the temperature at the outlet end of which was aboufit155F., to a final moisture content of about 3 to about 6% by We g Theremarkable utility of the methods and compositions of this invention ineffecting saving in veneer is obvious from the table.

EXAMPLE 5 Six walnut veener sheets of the same type as those describedin Example 1 were treated in the same manner as that described inExample 1, with an aqueous solution of hydroxyacetic acid and ureaidentical with that described in Example 1 and under the same conditionsof time and temperature as those described in Example 1. Subsequent tothe drying operation it was found that the average thickness of thetreated sheets was about 0.0334 inch.

Six control sheets identical with the six sheets as above-described inthis example were not treated with the composition of this invention butwere dried in precisely the same manner as the treated sheets. Theaverage thickness of the control sheets subsequent to drying was about0.0326 inch.

t is obvious from the foregoing that the sheets treated in accordancewith this invention demonstrated, subsequent to drying, an averagethickness about 0.002 inch greater than that of the untreated controlsheets. Assuming a conventional veneer thickness of about 0.033 inch, anextra veneer sheet is obtained through the practice of this inventionfor every 16 or 17 sheets produced and treated. The economicsignificance of the invention is thus readily apparent.

EXAMPLE 6 A group of 12 sheets of poplar having dimensions of about 10"x 24" and a like group of sheets of gum both having a thickness of of aninch were treated with the aqueous solution of Example 2. Subsequent totreatment the veneer sheets contained urea and hydroxyacetic acid in anamount equal to about 18% by weight of the sheets. The sheets containedabout 50% by Weight of water before treatment. The sotreated sheets werethereafter dried in a veneer dryer to a moisture content of about 5%.Simultaneously, substantially identical groups of untreated controlsheets of poplar and gum were dried.

After storage of both the treated and untreated dried sheets in achamber in which the relative humidity was maintained at about 30% atabout 26 C. for a period of about .3 days, constant weight was reachedand both the control and the treated sheets were exposed to air ofrelative humidity at 38 C. for a period of about 3 days. The treated gumsheets averaged 7.47% increase in weight whereas the untreated sheetsaveraged 7.59% increase in weight. The treated poplar sheets averaged7.43% increase in weight whereas the untreated poplar sheets averaged7.34% increase in weight.

This example clearly demonstrates that the compositions of thisinvention do not substantially increase the hydroscopicity of the woodtreated.

EXAMPLE 7 Example 2 was repeated with the exception that theurea-hydroxyacetic acid solution utilized was neutralized by addingammonium hydroxide in an amount requisite to raise the pH to about 6 or7, thereby forming a mixture of urea, hydroxyacetic acid, and theammonium salts of hydroxyacetic acid.

The results obtained are substantially identical with those described inExample 2.

Like results are obtained when alkali metal hydroxides, includingsodium, potassium, lithium hydroxides and carbonates, are utilized.

In some industrial drying operations the wood sheets may be carried bymetallic members which contact the wood to be dried. Under somecircumstances, this may give rise to some discoloration of the woodsurfaces. This problem may be obviated by predrying the surfaces onl ofthe wood with a stream of warm air prior to placing the wood in contactwith the metallic members of the drying apparatus.

I claim:

1. A composition for treating wood to reduce the volumetric shrinkage ofthe wood when the moisture content thereof is lowered, consistingessentially of urea and a material selected from the group consisting ofhydroxyacetic acid and the alkali metal and ammonium salts ofhydroxyacetic acid, said composition containing from about 0.08 to about0.30 parts by weight of said material per part of urea.

2. The composition of claim 1 containing from about 0.16 to about0.20'part by weight of said material per part by weight of urea.

3. An aqueous solution containing from about to about 70% by weight ofthe composition of claim 1, said solution having a pH not greater thanabout 7.

4. An aqueous solution containing from about 10% to about 45% by Weightof the composition of claim 1, said solution having a pH not greaterthan about 7.

5. An aqueous solution containing from about to about by weight of thecomposition of claim 1, said solution having a pH not greater than about'7.

6. An aqueous solution containing from about to about 75% by weight ofthe composition of claim 1, said solution having a pH not greater thanabout '7.

7. A composition for treating wood to reduce the volumetric shrinkage ofthe wood when the moisture content thereof is lowered, consistingessentially of from about 4 to about '7 parts by weight of crystallineurea, about 0.6 to about 1.2 parts by weight of a material selected fromthe group consisting of hydroxyacetic acid and the alkali metal andammonium salts of hydroxyacetic acid, and from about 0.4 to about 0.2part of water by weight, said composition having a pH not greater thanabout '7.

8. A composition for treating wood to reduce the volumetric shrinkage ofthe wood when the moisture content thereof is lowered comprising.

a mixture of urea and a material selected from the group consisting ofthe alkali metal and ammonium salts of hydroxyacetic acid, saidcomposition being characterized, when in aqueous solution, by a pH notgreater than about 7, and con- 8 taining from about 0.08 to about 0.30part of said material per part of urea.

9. The method which comprises impregnating a substantial portion of thewood with the composition of claim 1, thereafter drying the surface onlyof the so-treated wood, and subsequently completing the drying of thewood.

10. The composition of claim 1 wherein said material is hydroxyaceticacid.

11. The composition of claim 1 wherein said material is the ammoniumsalt of hydroxyacetic acid.

12. A process for reducing the volumetric shrinkage of wood when themoisture content thereof is lowered which comprises impregnating asubstantial portion of the wood, the shrinkage of which is to bereduced, with the composition of claim 1 and thereafter lowering themoisture content of the impregnated wood.

13. A process for reducing the volumetric shrinkage of wood when themoisture content thereof is lowered which comprises impregnating asubstantial portion of the wood, the shrinkage of which is to bereduced, with the composition of claim 2 and thereafter lowering themoisture content of the impregnated wood.

14. A process for reducing the volumetric shrinkage of wood when themoisture content thereof is lowered which comprises impregnating asubstantial portion of the wood, the shrinkage of which is to bereduced, with the composition of claim 5 and thereafter lowering themoisture content of the impregnated wood.

15. A process for reducing the volumetric shrinkage of wood when themoisture content thereof is lowered which comprises impregnating asubstantial portion of the wood, the shrinkage of which is to bereduced, with the composition of claim 6 and thereafter lowering themoisture content of the impregnated wood.

16. A process for reducing the volumetric shrinkage of wood when themoisture content thereof is lowered which comprises impregnating asubstantial portion of the wood, the shrinkage of which is to bereduced, with the composition of claim '7 and thereafter lowering themoisture content of the impregnated Wood.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,423,556 Feibelmann July 8, 1947 2,449,785 Lippman Sept. 21,1948 2,500,954 Loughborough Mar. 21, 1950 OTHER. REFERENCES ChemicalSeasoning, Rept. No. 1, West Coast Lumbermens Assn., Seattle,Washington, 1943, page 17.

1. A COMPOSITION FOR TREATING WOOD TO REDUCE THE VOLUMETRIC SHRINKAGE OFTHE WOOD WHEN THE MOISTURE CONTENT THEREOF IS LOWERED, CONSISTINGESSENTIALLY OF UREA AND A MATERIAL SELECTED FROM THE GROUP CONSISTING OFHYDROXYACETIC ACID AND THE ALKALI METAL AND AMMONIUM SALTS OFHYDROXYACETIC ACID, SAID COMPOSITION CONTAINING FROM ABOUT 0.08 TO ABOUT0.30 PARTS BY WEIGHT OF SAID MATERIAL PER PART OF UREA.
 9. THE METHODWHICH COMPRISES IMPREGNATING A SUBSTANTIAL PORTION OF THE WOOD WITH THECOMPOSITION OF CLAIM 1, THEREAFTER DRYING THE SURFACE ONLY OF THESO-TREATED WOOD, AND SUBSEQUENTLY COMPLETING THE DRYING OF THE WOOD.